United States Environmental Protection Agency (EPA) regulations limit the amount of particulate matter, measured in grams per hour, that may be emitted by a woodstove or wood-fired boiler. Catalytic converters are frequently used to clean wood-fired exhaust. A catalytic converter essentially burns unburned fuel (smoke) from the fire before it exits a flue (smoke stack).
Conventional catalytic converters often include ribbons of metal, typically thin (such as about 0.004 inch thick) stainless steel, that are coated with one or more catalysts. The ribbons are often corrugated or “herringbone” shaped and then wound into spirals or folded back and forth. The wound or folded ribbons are then bound, such as by stainless steel rings, to maintain a desired shape (typically round) and diameter (slightly less than the inside diameter of a smoke stack). Each ring has an upper lip and a lower lip to prevent the wound or folded ribbon from sliding out of the ring. A ring with one or more such ribbons is referred to herein as a packaged catalytic converter.
One or more such packaged catalytic converters may be placed inside a stack, such that the axis of each ring is aligned with the central axis of the stack. The ribbons are oriented such that their surfaces are parallel with the axis of the stack and, therefore, with the direction of flow of the smoke.
The outside diameter of a packaged catalytic converter is typically slightly less than the inside diameter of a stack to facilitate installing the catalytic converter into the stack and to accommodate thermal expansion of the ring. Sometimes the gap between the outside diameter of the ring and the inside diameter of the stack is packed with a compressible heat-resistant sealant. Consequently, smoke is prevented from passing around the packaged catalytic converter and, instead, passes over the surfaces of the ribbons. The sealant typically does not, however, provide much structural fixation of the ring to the stack.
A typical packaged catalytic converter weighs about 5 pounds, although the weight can vary, based on its diameter, length, material thickness, etc. Nevertheless, a typical packaged catalytic converter has a substantial mass packed into a relatively small volume, and the package (ring) has a hard outer surface.
Occasionally, small or large explosions occur within wood-burning appliances, particularly in gasification appliances. Such an explosion can propel a packaged catalytic converter out of the stack. Essentially, the stack becomes a canon barrel, and the packaged catalytic converter becomes a projectile. Catalytic converter packages have been known to be projected about 25 feet into the air above a wood-burning appliance. A hot catalytic converter package ejected from a stack can cause a fire if it lands on dry grass, a wood shingled roof or other flammable material. Furthermore, a falling catalytic converter can injure a person or animal or cause impact damage to property. Catalytic converter packages disposed within stacks can, therefore, pose safety problems.